Light emitted from natural light sources such as the sun is generally non-polarized (randomly polarized). Light emitted from artificial light sources such as a lamp is also generally non-polarized. A polarizing plate can extract polarized light (linearly, circularly or elliptically polarized light) from non-polarized light. The extracted polarized light can be used in various optical devices. For example, a liquid crystal display widely used nowadays is an instrument that utilizes characters of polarized light to display an image.
The polarizing plates in a broad sense include linearly, circularly and elliptically polarizing plates. However, the term of the polarizing plate usually (in a narrow sense) means only a linearly polarizing plate. In the present specification, the polarizing plate means a linearly polarizing plate, which is the basic polarizing plate.
A linearly polarizing plate generally used is a light-absorbing polarizing element consisting of a polyvinyl alcohol film. A polyvinyl alcohol film is stretched, and the film adsorbs iodine or a dichromatic dye to prepare a polarizing element. The stretched polarizing element has a transmission axis (polarizing axis) perpendicular to the stretching direction.
The light-absorbing polarizing element transmits only a light component polarized parallel to the polarizing axis, and absorbs a perpendicularly polarized component. There-fore, usable light through the element is theoretically 50% or less (practically, much less than 50%) of the original light.
A light-scattering polarizing element has been proposed in place of the light-absorbing polarizing element to increase the amount of usable light (efficiency of light). The light-scattering polarizing element also transmits only the light component polarized parallel to the polarizing axis, but scatters forward or backward the perpendicularly polarized component. The light-scattering polarizing element can improve the efficiency of light by using the scattered light.
The light-scattering polarizing element is described in Japanese Patent Provisional Publication Nos. 8(1996)-76114, 9(1997)-274108, 9(1997)-297204, Japanese Patent Publication Nos. 11(1999)-502036, 11(1999)-509014, and U.S. Pat. Nos. 5,783,120, 5,825,543 and 5,867,316.